Articles used outside have a protective film or adhesive sheet having such protective film layer on surfaces, in order to protect the surfaces from many stains and to keep appearance good. The protective film has generally been formed from synthetic resin having water repellent function or material containing fluorine-containing substance or silicon-containing substance for enhancing water repellent properties. However, it has been reported that the materials having water repellent function have the following defects:
(1) In case of hydrophilic stain substances, such as dirt, sand dust, dust or the like, the protective film has water repellency on the surface and water easily falls down as water drops from the surface. Some water drops, however, remain on the surface without falling down and are dried up thereon to form dryup marks, which attract stain substance and form dot-shape stains. It is also very difficult to remove the stains once remaining on the surface by natural cleaning function, such as rain, because of hydrophilic nature of such stains.
(2) In case of lipophilic stain substances, such as exhausted gas, smoke dust or the like, the protective film has relatively high lipophilic nature and easily attracts lipophilic stain substances.
In order to improve the above defects, it has been recently proposed that the surfaces are made not lipophilic but hydrophilic, that is lowering water contact angle. The hydrophilic surface allows hydrophilic stain substances to easily adhere thereon, but the substances are removed away by natural cleaning function, such as rain. The lipophilic stain substances keep away from the surface, because they are difficult to adhere onto the hydrophilic surface.
As explained above, the mechanism of hydrophilic surface is very clear and reasonable, but it is not so easy to make the protective surface hydrophilic. The protective film is generally formed from synthetic polymeric material that is not hydrophilic but lipophilic, as general. The synthetic polymeric material is made hydrophilic by introducing hydrophilic groups into polymer molecules, but if a coating formed from the synthetic material is hydrophilic as a whole, the coating itself shows poor durability and poor water resistance. Accordingly, what has presently been proposed and studied as simple and effective method is that an agent having hydrophilic nature or an agent imparting hydrophilic nature (hereafter called as “hydrophilic stain resistant agent” in the present invention) is added in the protective film.
The “hydrophilic stain resistant agent” called herein is acknowledged not as a general surfactant, but as a specific additive. An agent having such surface activity that has been known to the art lowers water resistance or durability of the coatings and deteriorates adhesion with an under layer. The hydrophilic stain resistant agent should be one that does not deteriorate coating performance and merely makes a surface of the coating hydrophilic, which has recently been studied.
For example, Japanese Kokai Publications 2001-89721 and Hei 8 (1996)-337771 disclose specific hydrophilic stain resistant agent, which is formulated into into a resin coating. The hydrophilic stain resistant agent has a relative low molecular weight and bleeds out onto a surface of the coating and is chemically decomposed to produce hydrophilic chemical species, which imparts hydrophilic nature to the coating. The hydrophilic stain resistant agent is locally present on the periphery of the surface after bleeding out and shows excellent stain resistance. The localization of the agent on the surface adversely causes significant reduction of stain resistance when the surface is cut off due to abrasion with time or due to coating operation.
On the other hand, Japanese Kokai Publication Hei 11 (1999)-267585 discloses a protective film in which a hydrophilic stain resistant agent is contained. The protective film is formed from a clear coating composition, which comprises (A) 30 to 90% by weight of a film forming resin, (B) 10 to 70% by weight of a curing agent, and (C) 1 to 50% by weight of the hydrophilic stain resistant agent, percentages being based on non-volatile weight ratio. The stain resistant agent (C) exemplifies organosilicate, organosilicate condense, inorganic oxide sol (e.g. aluminum oxide sol, silicate oxide sol, zirconium oxide sol, antimony oxide sol or the like). The clear coating composition is formed from a combination of a curable resin and an inorganic oxide type stain resistant agent and imparts protective effects, such as stain resistance or weather resistance, to an article coated with the composition.
Another coating composition having similar effects is also proposed in Japanese Kokai Publication Hei 9 (1997)-302257. The inorganic oxide type stain resistant agent, however, is trapped within the resin matrix of the cured resin and does not show sufficient hydrophilic nature, e.g. water contact angle of less than 70 degrees, during initial use (within one month after coating), contrary to the surface bleed type stain resistant agent. In this case, the hydrophilic nature of the coated film would become sufficient after passing a considerable time, especially after about half a year from starting use and enhance stain resistance, but it is difficult to impart stain resistance during the initial use. It is believed that the late revelation of hydrophilic nature is caused by exposure of the stain resistant agent to the surface due to abrasion of the coated resin and the like, or by enhancement of level of hydrophilic nature due to chemical change of the stain resistant agent.